JP6891886B2 - Lithium titanate powder and active material for electrodes of power storage devices, and electrode sheets and power storage devices using them. - Google Patents

Description

The present invention relates to lithium titanate powder suitable as an electrode material for a power storage device, an active material material using this lithium titanate powder, and a power storage device using this active material material for a positive electrode sheet or a negative electrode sheet. is there.

In recent years, various materials have been studied as electrode materials for power storage devices. Among them, lithium titanate is attracting attention as an active material material for power storage devices for electric vehicles such as HEVs, PHEVs, and BEVs because it has excellent input / output characteristics when used as an active material material.

Since it is not uncommon for the temperature inside an automobile to exceed 60 ° C. in the summer, it is required that the power storage device for an electric vehicle has no safety problem and its performance does not deteriorate even at a high temperature. However, when a power storage device containing lithium titanate operates at such a high temperature, the reaction of lithium titanate with the electrolytic solution tends to proceed, which causes gas to be generated and the power storage device to expand, so that the safety of the power storage device is safe. Problems arise. Further, at a high temperature, there are problems such as a large decrease in capacity and a large increase in resistance when charging and discharging are repeated. Therefore, it is desired to develop lithium titanate in which gas generation, capacity reduction, and resistance increase are suppressed during high-temperature operation of the power storage device.

In Patent Document 1, when the temperature is raised to 60 ° C. to 900 ° C. by thermal decomposition gas chromatograph mass spectrometry, the total amount of water generated is 1500 wtppm or less, and the total amount of carbon dioxide generated is 2000 wtppm or less. Titanium composite oxides are disclosed. And, even if the specific surface area is increased, this lithium titanium composite oxide can suppress an increase in the amount of adsorbed gas and the amount of solvent for preparing the coating liquid, and enhances the safety of the lithium ion secondary battery. It is said that it can contribute to.

On the other hand, from the viewpoint of forming a homogeneous electrode, lithium titanate powder having specific powder physical characteristics, which is synthesized by firing after granulation, is preferable. For example, in Patent Document 2, primary particles are aggregated to form spherical secondary particles having a particle size of 1 to 50 μm, a specific surface area of 0.5 to 10 m ² / g, and a main component of Li _{4 /.} A lithium-titanium composite oxide comprising ₃ Ti _5/3 O _{4 is disclosed.} The handling and coatability on the current collector are good, and the non-aqueous electrolyte secondary battery using this has a high initial discharge capacity, and it is said that the capacity variation width during repeated use is small.

For example, Patent Document 3 discloses a granulated body of lithium titanate having a degree of pulverization Zd (D50 before pulverization / D50 after pulverization) of 2 or more when a load of 35 MPa is applied to 1 g of a sample for 1 minute. .. It is said that this lithium titanate granulated body is easily pulverized by pulverization before or during mixing of the coating slurry, facilitates dispersion, and strengthens the binding to the current collector.

Further, Patent Document 4 describes an agglomerated granular lithium composite oxide formed by aggregating a large number of specific fine powders of lithium nickel cobalt oxide, having an angle of repose of 45 to 65 ° and compression per grain. A cohesive granular lithium composite oxide characterized by having a breaking strength (crushing strength) of 0.1 to 1.0 gf is disclosed.

Further, Patent Document 5 describes that the primary particles of the active material are agglomerated secondary particles, the primary particles of the active material contain a specific niobium composite oxide, and the compressive fracture strength (crush strength) is 10 MPa or more. An active material for a battery including secondary particles and a carbon material phase formed on at least a part of the surface of the secondary particles is disclosed.

Japanese Unexamined Patent Publication No. 2014-1110 Japanese Unexamined Patent Publication No. 2001-192208 International Publication 2014/196462 Japanese Unexamined Patent Publication No. 2001-80920 Japanese Unexamined Patent Publication No. 2015-888467

The lithium titanate powder of Patent Document 1 in which the water content is suppressed can suppress the generation of high-temperature gas as compared with the lithium titanate powder having a large water content by suppressing the water content, but is a high-temperature gas. In addition to the fact that the suppression of generation is not always sufficient, it is not possible to suppress the capacity decrease and resistance increase during the high temperature charge / discharge cycle. Further, the lithium titanate powder of Patent Document 2 has better handling at the time of electrode fabrication as compared with the lithium titanate powder produced without undergoing the granulation step, but the electrode density is lower and the electrode volume per electrode volume. In addition to the smaller capacity, none of gas generation, capacity reduction and resistance increase cannot be suppressed. Further, the lithium titanate powder of Patent Document 3 has better dispersibility in the binder as compared with the lithium titanate powder produced by firing at a high temperature after granulation, but the lithium titanate of Patent Document 2 Like powder, none of gas generation, volume reduction and resistance increase can be suppressed.

Further, Patent Document 4 discloses that the compressive strength of lithium nickel cobalt oxide is 0.1 to 1.0 gf (7.7 to 77 MPa). This Patent Document 4 discloses that the compressive strength when the lithium nickel cobalt oxide particles are broken, that is, the compressive fracture strength (crush strength) is within the above range. Then, according to Patent Document 4, by setting the compressive fracture strength (crush strength) in the above range, the agglomerates are broken by a slight pressure to become a fine powder, and further, the fine powder is positive. It is only disclosed that the initial discharge capacity can be high and the capacity retention rate of the discharge capacity can be increased by being able to be evenly distributed on the finest surface. No findings have been presented regarding the suppression of volume reduction.

Further, Patent Document 5 discloses secondary particles in which primary particles of niobium composite oxide are aggregated and have a compressive fracture strength (crush strength) of 10 MPa or more. According to this Patent Document 5, if the compressive fracture strength (crush strength), which is the strength when the secondary particles collapse, is less than 10 MPa, the secondary particles collapse during dispersion, and the binding property of the electrode becomes poor. The decrease and the electron conductivity are significantly reduced. On the other hand, by setting the compressive fracture strength (crush strength) to 10 MPa or more, it becomes possible to significantly suppress the particle decay during dispersion and the electron conduction. It is disclosed that good charge / discharge life performance can be realized because the path does not easily collapse. However, Patent Document 5 does not show any knowledge about gas generation at high temperature and suppression of capacity decrease during high temperature charge / discharge cycle.

Therefore, in the present invention, when applied as an electrode material for a power storage device, it is possible to suppress gas generation at high temperature and capacity decrease during high temperature charge / discharge cycle, and also suppress resistance increase during high temperature charge / discharge cycle. It is an object of the present invention to provide a lithium titanate powder, an active material, an electrode sheet of a power storage device containing them, and a power storage device using the electrode sheet.

As a result of various studies to achieve the above object, the present inventors have found that it is a lithium titanate powder containing secondary particles in which primary particles are aggregated, has a specific degree of aggregation, and is released under a specific temperature condition. A storage device in which a lithium titanate powder having a small amount of water content and an average compression strength of 10% of the contained secondary particles in a specific range was found and the lithium titanate powder was applied as an electrode material, The present invention has been completed by finding that, in addition to the small amount of gas generated at high temperature, the capacity decrease and resistance increase during the high temperature charge / discharge cycle are small. That is, the present invention relates to the following matters.

(1) _{A ratio of lithium titanate powder containing Li 4} Ti ₅ O ₁₂ as a main component, which contains secondary particles in which primary particles made of lithium titanate are aggregated, and is calculated from the specific surface area obtained by the BET method. When the surface area equivalent diameter is D _BET and the volume median particle size is D50, D _BET is 0.03 μm or more and 0.6 μm or less, D50 is 3 μm or more and 40 μm or less, and the ratio of D50 _{to D BET is D50.} / D _BET (μm / μm) is 20 or more and 350 or less, the water content (25 ° C to 350 ° C) measured by the curl Fisher method is 600 ppm or less, and the average 10% compression strength of the secondary particles is 0. .Lithium titanate powder for electrodes of power storage devices, characterized by 1 MPa or more and 3 MPa or less.

(2) The lithium titanate powder for electrodes of the power storage device according to (2), wherein the compressive fracture strength (crush strength) is not detected.

(3) The lithium titanate powder for electrodes of the power storage device according to (1) or (2), wherein the water content (200 ° C. to 350 ° C.) measured by the Karl Fischer method is 150 ppm or less.

(4) The lithium titanate powder for an electrode of a power storage device according to any one of (1) to (3), wherein D _max is 53 μm or less when the maximum volume particle size is D _max.

(5) The lithium titanate powder for an electrode of a power storage device according to any one of (1) to (4), wherein the secondary particles have an average circularity of 90% or more.

(6) The lithium titanate powder for an electrode of a power storage device according to any one of (1) to (5), wherein the average 10% compression strength of the secondary particles is 0.1 MPa or more and 1 MPa or less.

(7) An active material material containing lithium titanate powder for electrodes of the power storage device according to any one of (1) to (6).

(8) An electrode sheet of a power storage device produced by using the active material material according to (7).

(9) A power storage device including the electrode sheet according to (8).

(10) A lithium ion secondary battery manufactured by using the active material material according to (7).

(11) A hybrid capacitor produced by using the active material according to (7).

(12) Ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one and 4-ethynyl-1,3-dioxolan-2-one. An electrolyte salt containing at least one lithium salt selected from _{LiPF 6} , LiBF ₄ , LiPO ₂ F ₂ , and LiN (SO ₂ F) ₂ was dissolved in a non-aqueous solvent containing one or more cyclic carbonates selected from the above. The power storage device according to (9), wherein a non-aqueous electrolyte solution is used.

(13) The non-aqueous electrolyte solution has a total electrolyte salt concentration of 0.5 M or more and 2.0 M or less, _{contains at least LiPF 6 as the electrolyte salt, and further contains LiBF 4} of 0.001 M or more and 1.0 M or less. , LiPO ₂ F ₂ , and LiN (SO ₂ F) _2. The power storage device according to (12), which is a non-aqueous electrolyte solution containing at least one lithium salt selected from LiN (SO 2 F) 2.

(14) The non-aqueous electrolyte solution includes one or more symmetrical chain carbonates selected from dimethyl carbonate, diethyl carbonate, dipropyl carbonate, and dibutyl carbonate, and methyl ethyl carbonate, methyl propyl carbonate, and methyl isopropyl carbonate. , Methylbutyl carbonate, and one or more asymmetric chain carbonates selected from ethylpropyl carbonate, according to (12) or (13).

According to the present invention, lithium titanate powder and an active material material, which are suitable as an electrode material for a power storage device in which gas generation is suppressed with little increase in resistance or decrease in capacity during a high temperature charge / discharge cycle of the power storage device, and active material materials thereof are included. It is possible to provide an electrode sheet of a power storage device and a power storage device using the electrode sheet.

Measured with the amount of gas generated during aging of an 800 mAh laminated battery prepared by setting the vacuum drying condition of the negative electrode using lithium titanate powder as an active material according to Examples and Comparative Examples at 80 ° C. for 2 hours, and the lithium titanate powder. It is a graph which showed the relationship between the water content (25 ° C. to 200 ° C.) and the water content (200 ° C. to 350 ° C.). Measured with the amount of gas generated during aging of an 800 mAh laminated battery prepared by setting the vacuum drying condition of the negative electrode using lithium titanate powder as an active material according to Examples and Comparative Examples at 150 ° C. for 12 hours, and the lithium titanate powder. It is a graph which showed the relationship between the water content (25 ° C. to 200 ° C.) and the water content (200 ° C. to 350 ° C.).

[Lithium titanate powder of the present invention]
The lithium titanate powder of the present invention is a lithium titanate powder _{containing Li 4} Ti ₅ O ₁₂ as a main component, and contains secondary particles in which primary particles made of lithium titanate are aggregated, and the ratio determined by the BET method. When the specific surface area equivalent diameter calculated from the surface area is D _BET and the volume median particle size is D50, D _BET is 0.03 μm or more and 0.6 μm or less, D50 is 3 μm or more and 40 μm or less, and D50. mean and _D ratio of _{BET D50 / D BET (μm /} μm) is 20 or more 350 or less, the amount of moisture measured by the Karl Fischer method (25 ° C. to 350 ° C.) is not more than 600 ppm, of the secondary particles It is a lithium titanate powder for an electrode of a power storage device, characterized in that the 10% compressive strength is 0.1 MPa or more and 3 MPa or less, and the compressive breaking strength (crushing strength) is not detected.

< _{Lithium titanate powder containing Li 4} Ti ₅ O ₁₂ as the main component>
The lithium titanate powder of the present invention contains Li ₄ Ti ₅ O ₁₂ as a main component and contains crystalline components and / or amorphous components other than _{Li 4} Ti ₅ O ₁₂ within the range in which the effects of the present invention can be obtained. Can be done. Lithium titanate powder of the present invention, among the diffraction peaks as measured by X-ray diffraction _method, the intensity of the main peak of Li ₄ Ti _{5 O 12,} the main due to the crystalline components other than Li ₄ Ti _{5 O 12 The ratio of the intensity of the main peak of Li 4} Ti ₅ O ₁₂ to the sum of the intensity of the peak and the maximum intensity of the halo pattern due to the amorphous component is preferably 90% or more, preferably 95% or more. Is even more preferred. The lithium titanate powder of the present invention may contain anatase-type titanium dioxide and rutile-type titanium dioxide, and Li ₂ TiO ₃ , which is lithium titanate having a different chemical formula, as the crystalline component due to the raw material at the time of its synthesis. is there. In the lithium titanate powder of the present invention, as the proportion of these crystalline components is smaller, the charge rate characteristics and charge / discharge capacity of the power storage device are further improved. Therefore, among the diffraction peaks measured by the X-ray diffraction method, Li _{4 When} the strength of the main peak of Ti ₅ O ₁₂ is 100, the strength of the main peak of anatase-type titanium dioxide, the main peak strength of rutile-type titanium dioxide, and the (-133) plane of _{Li 2} TiO _{3 are equivalent.} It is particularly preferred that the sum of the peak intensity and the intensity corresponding to the main peak of _{Li 2} TiO ₃ calculated by multiplying it by 100/80 is 5 or less. _Here, the main peak of Li ₄ Ti _{5 O 12,} attributed to _Li 4 _Ti of 5 _{O 12} (111) plane in PDF card 00-049-0207 of ICDD (PDF2010) (2θ = 18.33 ) diffraction It is a peak corresponding to the peak. The main peak of anatase-type titanium dioxide is a peak corresponding to the diffraction peak belonging to the (101) plane (2θ = 25.42) in the PDF card 01-070-6826. The main peak of rutile-type titanium dioxide is a peak corresponding to the diffraction peak attributable to the (110) plane (2θ = 27.44) in the PDF card 01-070-7347. Peak corresponding to the diffraction peak with the peak corresponding to Li ₂ TiO ₃ of (-133) plane attributed to _Li 2 TiO ₃ of (-133) plane in PDF card 00-033-0831 (2θ = 43.58) The main peak of Li ₂ TiO ₃ is a peak corresponding to the (002) plane. "ICDD" is an abbreviation for International Center for Diffraction Data, and "PDF" is an abbreviation for Powder Diffraction File.

<Secondary particles in which primary particles are aggregated>
The lithium titanate powder of the present invention contains secondary particles formed by aggregating primary particles made of lithium titanate. The lithium titanate powder of the present invention contains secondary particles formed by aggregating primary particles made of lithium titanate, but some of them do not form secondary particles and are primary. It may be in the form of the particles themselves.

<Diameter equivalent to specific surface area ( _DBET )>
_{The specific surface area equivalent diameter (DBET} ) of the lithium titanate powder calculated from the specific surface area obtained by the BET method of the present invention is 0.03 μm or more and 0.6 μm or less. _{The DBET} of the lithium titanate powder of the present invention is an index related to the size of the primary particles. _{The method for calculating the DBET} of the lithium titanate powder of the present invention will be described in Examples described later. From the viewpoint of improving the charge / discharge rate characteristics, the _DBET is preferably 0.4 μm or less. From the viewpoint of suppressing the generation of gas in the power storage device, the _DBET is preferably 0.1 μm or more.

<Medium volume particle size (D50)>
The volume median particle diameter (D50) of the lithium titanate powder of the present invention is 3 μm or more and 40 μm or less. The D50 of the lithium titanate powder of the present invention is an index related to the average particle size of the secondary particles. Here, D50 means a particle size in which the cumulative volume frequency calculated by the volume fraction obtained by laser diffraction / scattering type particle size distribution measurement is integrated from the smallest particle size to 50%. The method for measuring D50 of the lithium titanate powder of the present invention will be described in Examples described later. From the viewpoint of improving the handling of the coating process and the like, the D50 is preferably 5 μm or more. The upper limit of D50 is not particularly limited, but is preferably 30 μm or less.

<D50 / D _BET >
_{The ratio of D50 to D BET} (D50 / D _BET (μm / μm)) of the lithium titanate powder of the present invention is 20 or more and 350 or less. The lithium titanate powder of the present invention contains secondary particles in which primary particles made of lithium titanate are aggregated. _{The D50 / D BET} of the lithium titanate powder of the present invention is an index related to the degree of aggregation of the primary particles with respect to the secondary particles. From the viewpoint of ease of forming secondary particles, _{the lower limit of D50 / D BET} is preferably 30 or more, and more preferably 40 or more. From the viewpoint of facilitating the formation of the electrode mixture layer in which the active material, the conductive agent and the binder are uniformly distributed _{, the upper limit of D50 / D BET} is preferably 250 or less, more preferably 150.

<Maximum volume particle size (D _max )>
The maximum volume particle size (maximum particle size, hereinafter referred to as “D _max ”) of the lithium titanate powder of the present invention is preferably 53 μm or less. D _max is obtained by laser diffraction / scattering type particle size distribution measurement. Here, D _max means a particle size in which the cumulative volume frequency becomes 100% by integrating from the smaller particle size. The measuring method will be described in Examples described later. D _max is more preferably 45 μm or less from the viewpoint of improving the handling of the coating process.

<Moisture content>
The water content (25 ° C. to 350 ° C.) measured by the Karl Fischer method of lithium titanate powder of the present invention is 200 ° C. by heating the lithium titanate powder of the present invention from 25 ° C. to 200 ° C. under nitrogen flow. The amount of water released from the lithium titanate powder of the present invention during the period from the start of heating to the completion of holding at 200 ° C. when the product was held at ° C. for 1 hour was measured by the Karl Fischer method, followed by the amount of water obtained. When the lithium titanate powder of the present invention was heated from 200 ° C. to 350 ° C. and held at 350 ° C. for 1 hour under nitrogen flow, the present invention was conducted between the start of heating at 200 ° C. and the completion of holding at 350 ° C. It is the amount of water obtained by totaling the amount of water released from the lithium titanate powder of the present invention measured by the Karl Fischer method, and the measuring method is paragraph 0124 <Moisture by the Karl Fischer method. It will be explained in detail in Measurement of quantity>. The water content (25 ° C. to 350 ° C.) of the lithium titanate powder of the present invention measured by the Karl Fischer method, that is, the water content at 25 ° C. to 350 ° C. is 600 ppm or less. When it is 600 ppm or less, when applied as an electrode material of a power storage device, the resistance increase and capacity decrease during the high temperature charge / discharge cycle of the power storage device are small, and gas generation can be suppressed. The water content (25 ° C. to 350 ° C.) measured by the Karl Fischer method includes both the water content physically adsorbed on the lithium titanate powder of the present invention and the water content chemically adsorbed. Usually, in lithium titanate powder, it is difficult to measure by the Karl Fischer method in the region above 350 ° C., and almost no water is detected by other methods (for example, pyrolysis gas chromatograph mass spectrometry). The amount of water (25 ° C. to 350 ° C.) measured by the Karl Fischer method is more preferably 500 ppm or less from the viewpoint of suppressing an increase in resistance, a decrease in capacity, and gas generation during a high-temperature charge / discharge cycle of the power storage device.

The water content (200 ° C. to 350 ° C.) measured by the Karl Fischer method of lithium titanate powder of the present invention is 350 ° C. from the start of heating at 200 ° C. among the water content (25 ° C. to 350 ° C.). It is the amount of water obtained by measuring the water released from the lithium titanate powder of the present invention by the Karl Fischer method until the retention of the above-mentioned water amount (25 ° C. to 350 ° C.). The same as the method for measuring (° C.)) will be described in detail in paragraph 0124 <Measurement of water content by Karl Fischer method>. The water content (200 ° C. to 350 ° C.) of the lithium titanate powder of the present invention measured by the Karl Fischer method, that is, the water content at 200 ° C. to 350 ° C. is preferably 150 ppm or less. The water contained in lithium titanate includes the physically adsorbed water and the chemically adsorbed water as described above, but the water present on the surface of both of them can be up to 200 ° C. It is presumed that most of them are desorbed and contained in the water content (25 ° C to 200 ° C) measured by the Karl Fischer method. In addition, since there is a step of drying the electrodes in the production of a normal power storage device, the amount of water (25 ° C. to 200 ° C.) measured by the Karl Fischer method (as in <Reference Experimental Example 1> described later) is this. It will be almost released in such a drying step. Therefore, it is considered that the water that affects the power storage device is not the surface of the lithium titanate particles, but the water that exists inside the particles that are difficult to remove in such a drying step. Therefore, it is considered that most of the water content existing inside the particles and substantially affecting the power storage device is contained in the water content (200 ° C. to 350 ° C.) measured by the Karl Fischer method. From the above viewpoint, the water content (200 ° C. to 350 ° C.) measured by the Karl Fischer method is more preferably 100 ppm or less. The lower limit of the water content (200 ° C. to 350 ° C.) measured by the Karl Fischer method is not particularly limited, and in some cases, it can be determined to be 0 ppm when it is below the detection limit of the measuring device. In some cases.

<Average compression strength of secondary particles 10%>
As described above, the lithium titanate powder of the present invention contains secondary particles formed by aggregating primary particles made of lithium titanate, and the average 10% compressive strength of such secondary particles is 0. .1 MPa or more and 3 MPa or less. When the average 10% compression strength of the secondary particles is 0.1 MPa or more, it is possible to reduce the resistance increase and capacity decrease during the high temperature charge / discharge cycle of the energy storage device when applied as an electrode material of the energy storage device. Here, setting the average 10% compression strength of the secondary particles to 3 MPa or less is also effective for increasing the density of the electrode mixture layer, that is, increasing the energy density. From the above viewpoint, the upper limit of the average 10% compression strength is more preferably 1 MPa or less. The average 10% compressive strength of the secondary particles is the average value of the compressive strength when compressed by 10% of the particle size to be measured, and specifically, a predetermined number contained in the lithium titanate powder of the present invention. For each of the secondary particles, the compression strength when compressed by 10% of the measured particle size measured by a compression test device was measured, and the average value was calculated for the 10% compression strength of the obtained predetermined number of secondary particles. The calculated and obtained average value of the compressive strength can be taken as the average compressive strength of 10% of the secondary particles constituting the lithium titanate powder of the present invention. A specific method for measuring the average compression strength of secondary particles of 10% will be described in Examples described later.

In the present invention, the secondary particles are composed of a plurality of primary particles aggregated, and the average 10% compression strength of the secondary particles (the average value of the compression strength at the time of 10% compression) is as described above. It is one of the indexes showing the mode of aggregation of primary particles, and shows the magnitude of stress required for 10% compression (the magnitude of stress required to reach a predetermined amount of deformation). Yes, it shows the characteristics of the secondary particles themselves. On the other hand, the compressive fracture strength (crush strength) disclosed in Patent Documents 4 and 5 described above is the strength at which the secondary particles themselves collapse, and specifically, under that strength, As an index indicating whether or not the shape of the next particle can be maintained, such compression fracture strength (crush strength) is defined in the present invention as having an average 10% compression strength (compression strength at 10% compression). It is completely different from the average value of) and usually does not correlate.

The lithium titanate powder of the present invention, which has an average compression strength of 10% in the above range, can be compressed in excess of 10%, but when the compression load is increased, the compression load is increased. Since the secondary particles can be plastically deformed in the compression direction accordingly, the secondary particles themselves do not collapse, and have substantially no compressive fracture strength (crush strength).

<Average circularity>
As described above, the lithium titanate powder of the present invention contains secondary particles formed by aggregating primary particles made of lithium titanate, but the average circularity of the contained secondary particles is 90% or more. It is preferable to have. When the average circularity is 90% or more, the lithium titanate powder is well dispersed when mixed with other electrode constituent materials such as a conductive agent to form a paint, and a mixture layer having a good degree of mixing with the conductive agent is formed. It is preferable because it is easy to do. The method for measuring the average circularity will be described in Examples described later. From the viewpoint of further enhancing the effect of the present invention, the average circularity is preferably 93% or more, more preferably 95% or more. The lithium titanate powder of the present invention is preferably composed of secondary particles having a circularity of 90% or more, but to the extent that the characteristics of the power storage device to which the lithium titanate powder of the present invention is applied are not affected. Lithium titanate powder particles other than secondary particles having a circularity of 90% or more (for example, secondary particles having a circularity of less than 90% or primary particles that are not secondary particles) may be included.

As in the present invention, the amount of water contained in the powder is small, and the average 10% compression strength of the secondary particles is within a specific range, so that the resistance of the power storage device increases and the capacity decreases significantly during the high-temperature charge / discharge cycle. It can be suppressed. On the other hand, even if the amount of water contained in the powder is small, if the powder is substantially composed of primary particles or if the secondary particles have an average compression strength lower than the range of the present invention, the resistance during the high temperature charge / discharge cycle The increase and capacity decrease will be large. Further, even if the secondary particles have an average compression strength of 10% higher than the range of the present invention, the resistance increase and the capacity decrease during the high temperature charge / discharge cycle become large. Further, even if the secondary particles have an average compression strength of 10% within the range of the present invention, if the amount of water contained in the powder is large, the amount of gas generated by the power storage device increases, and the resistance and capacity during the high temperature charge / discharge cycle increase. The decrease will be large.

The reason for this cannot be overlooked, but we will consider it as follows. Even if it does not have the required degree of aggregation of the present invention (that is, D50 / D _BET is not within the specified range of the present invention), or even if it has the specified degree of aggregation of the present invention, the average compressive strength of the secondary particles is 10%. However, the lithium titanate powder smaller than the present invention cannot form an electrode mixture layer in which the active material, the conductive agent and the binder are uniformly distributed, so that the capacity decrease and the resistance increase during the high temperature charge / discharge cycle are suppressed. It is thought that it cannot be done.

On the other hand, even if the particles have a predetermined degree of aggregation in the present invention, the secondary particles are less likely to be deformed by stress in the lithium titanate powder in which the average compressive strength of the secondary particles is larger than the range of the present invention. When gas is generated from the surface of the primary particles constituting such secondary particles during the high temperature charge / discharge cycle, the discharge path is not secured, so that the discharged gas is immediately discharged from the secondary particles. However, it is considered that the substantial reaction area between the active material and the electrolytic solution becomes smaller. Alternatively, it is conceivable that the gas that has remained inside the secondary particles until the volume becomes relatively large is discharged from the secondary particles at once, and the adhesion between the mixture layer and the current collector is reduced. It is considered that when such a phenomenon occurs, it becomes impossible to suppress the capacity decrease and the resistance increase during the high temperature charge / discharge cycle.

The lithium titanate powder of the present invention is an active material, a conductive agent, and a binder because it has secondary particles having a specific degree of aggregation and an average compression strength of 10% in a specific range in the lithium titanate powder having a small water content. In addition to forming an electrode mixture layer in which the coating material is evenly distributed, the amount of gas generated during the high-temperature charge / discharge cycle is small, and the gas discharged from the surface of the primary particles constituting the secondary particles is the same. It is considered that the particles are discharged little by little from the secondary particles and the electrode mixture layer without staying in the vicinity. As a result, it is presumed that in addition to suppressing gas generation and capacity reduction during the high-temperature charge / discharge cycle, resistance increase during the high-temperature charge / discharge cycle is also suppressed. It should be noted that such a problem in a high temperature environment is a problem peculiar to lithium titanate, and in lithium titanate, a problem that did not occur at room temperature newly occurs in a high temperature environment. The present invention effectively solves such a problem. Further, for example, even if the charge / discharge cycle characteristics at room temperature are good, the charge / discharge cycle characteristics at high temperature are not necessarily good, as can be seen from the disclosure of Japanese Patent Application Laid-Open No. 2013-20909, for example. As is clear, those skilled in the art can easily understand it.

[Method for producing lithium titanate powder of the present invention]
Hereinafter, an example of the method for producing the lithium titanate powder of the present invention will be described separately for the raw material preparation step, the firing step, and the post-treatment step, but the method for producing the lithium titanate powder of the present invention is limited thereto. Not done.

<Raw material preparation process>
The raw material of the lithium titanate powder of the present invention comprises a titanium raw material and a lithium raw material. As the titanium raw material, titanium compounds such as anatase-type titanium dioxide and rutile-type titanium dioxide are used. It is preferable that it easily reacts with the lithium raw material in a short time, and from that viewpoint, anatase-type titanium dioxide is preferable. In order to sufficiently react the raw materials by firing in a short time, the volume median particle diameter (average particle diameter, D50) of the titanium raw material is preferably 2 μm or less.

As the lithium raw material, lithium compounds such as lithium hydroxide monohydrate, lithium oxide, lithium hydrogen carbonate, and lithium carbonate are used.

In the present invention, when the mixture composed of the above raw materials is fired in a short time, the mixed powder constituting the mixture is subjected to D95 in the particle size distribution curve measured by a laser diffraction / scattering type particle size distribution measuring machine before firing. It is preferable to prepare so that the value is 5 μm or less. Here, D95 is a particle size in which the cumulative volume frequency calculated by the volume fraction is 95% in total from the smallest particle size.

As a method for preparing the mixture, the following methods can be adopted. The first method is a method in which the raw materials are mixed and then pulverized at the same time as mixing. The second method is a method in which each raw material is pulverized until the D95 after mixing becomes 5 μm or less, and then these are mixed or mixed while being lightly pulverized. The third method is a method in which each raw material is produced into a powder composed of fine particles by a method such as crystallization, classified as necessary, and these are mixed or mixed while being lightly crushed. Among them, in the first method, the method of pulverizing at the same time as mixing the raw materials is an industrially advantageous method because there are few steps. At the same time, a conductive agent may be added.

In any of the first to third methods, the method of mixing the raw materials is not particularly limited, and either a wet mixing method or a dry mixing method may be used. For example, a Henschel mixer, an ultrasonic disperser, a homomixer, a mortar, a ball mill, a centrifugal ball mill, a planetary ball mill, a vibrating ball mill, an attritor type high-speed ball mill, a bead mill, a roll mill and the like can be used.

When the obtained mixture is a mixed powder, it can be directly used in the next firing step. In the case of a mixed slurry composed of mixed powder, the mixed slurry can be dried by a rotary evaporator or the like and then subjected to the next firing step. When firing is performed using a rotary kiln furnace, the mixed slurry can be provided in the furnace as it is.

<Baking process>
The resulting mixture is then fired. From the viewpoint of increasing the specific surface area of the powder obtained by firing and increasing the crystallite diameter, firing at a high temperature and in a short time is preferable. From such a viewpoint, the maximum temperature at the time of firing is preferably 1000 ° C. or lower, more preferably 950 ° C. or lower, and further preferably 900 ° C. or lower. From the viewpoint of reducing the proportion of the specific impurity phase and increasing the crystallinity of lithium titanate, the maximum temperature at the time of firing is preferably 800 ° C. or higher, more preferably 810 ° C. or higher. Similarly, from the above viewpoint, the holding time at the maximum temperature at the time of firing is preferably 2 to 60 minutes, more preferably 5 to 45 minutes, and further preferably 5 to 30 minutes. When the maximum temperature during firing is high, it is preferable to select a shorter holding time. Similarly, from the viewpoint of increasing the crystallite diameter obtained by firing, it is preferable to particularly shorten the residence time at 700 to 800 ° C. in the temperature raising process during firing, for example, within 15 minutes.

The firing method is not particularly limited as long as it can be fired under such conditions. Examples of the firing method that can be used include a fixed bed type firing furnace, a roller hearth type firing furnace, a mesh belt type firing furnace, a fluidized bed type firing furnace, and a rotary kiln type firing furnace. However, in the case of efficient firing in a short time, a roller hearth type firing furnace, a mesh belt type firing furnace, and a rotary kiln type firing furnace are preferable. When using a roller hearth type firing furnace or a mesh belt type firing furnace in which the mixture is placed in a bowl and fired, the quality of lithium titanate obtained by ensuring the uniformity of the temperature distribution of the mixture during firing is constant. In order to achieve this, it is preferable to reduce the amount of the mixture contained in the pot to a small amount.

The rotary kiln type firing furnace does not require a container to store the mixture, and can be fired while continuously charging the mixture, and the heat history to the object to be fired is uniform, and homogeneous lithium titanate can be obtained. Therefore, it is a particularly preferable firing furnace for producing the lithium titanate powder of the present invention.

The atmosphere at the time of firing is not particularly limited regardless of the firing furnace as long as the atmosphere can remove the desorbed water and carbon dioxide gas. Usually, the air atmosphere uses compressed air, but an oxygen, nitrogen, or hydrogen atmosphere may also be used.

<Post-treatment process>
Examples of a method for reducing the amount of water contained in the lithium titanate powder and imparting an average compression strength of 10% in a specific range to the secondary particles include the following post-treatment steps.

That is, the lithium titanate powder after firing obtained as described above does not need to be pulverized so as to destroy the particles, although it has slight agglutination. Therefore, it is necessary as a post-treatment step. Depending on the situation, crushing and classification may be performed to the extent that agglutination is resolved. The high crystallinity of the lithium titanate powder after firing is maintained even after that, if only crushing to the extent that the agglomeration is disintegrated without crushing.

The lithium titanate powder of the present invention may be coated. Any material may be used for coating, but acidic organic compounds and inorganic compounds are preferable, and specifically, acetic acid, oxalic acid, citric acid, aluminum acetate, aluminum fluoride or aluminum sulfate. Etc. are preferable.

In order to make the lithium titanate powder of the present invention into secondary particles in which primary particles are aggregated, granulation may be performed as a post-treatment step. Any method may be used for granulation as long as secondary particles can be produced, but it is preferable because a large amount of spray dryer can be processed.

Heat treatment may be performed as a post-treatment step in order to reduce the amount of water contained in the lithium titanate powder and to keep the average 10% compressive strength of the secondary particles in an appropriate range. Any method or condition may be used for the heat treatment as long as the amount of water can be reduced and the average compression strength is increased by 10%, but from the viewpoint of reducing the water content, the heat treatment temperature is preferably 200 ° C. or higher. Further, from the viewpoint of increasing the average 10% compression strength of the secondary particles to an appropriate range, the heat treatment temperature is preferably 300 ° C. or higher. From the viewpoint of keeping the average 10% compression strength of the secondary particles from exceeding an appropriate range, the heat treatment temperature is preferably 600 ° C. or lower. When the heat-treated powder is exposed to the atmosphere as it is, the amount of water contained in the powder increases. Therefore, it is preferable to handle the powder in a dew point-controlled environment during cooling in the heat treatment furnace and after the heat treatment. The powder after the heat treatment may be classified as necessary in order to bring the secondary particles into a desired maximum particle size range. When the powder is put out in an environment where the dew point is not controlled, it is preferable to put the lithium titanate powder of the present invention in an environment outside the dew point control after sealing it with an aluminum laminate bag or the like. Even under dew point control, if the lithium titanate powder after heat treatment is crushed, it becomes easier to take in water from the crushed surface and the amount of water contained in the powder increases. Therefore, do not crush when heat treatment is performed. Is preferable.

[Active material]
The active material material of the present invention contains the lithium titanate powder. It may contain one kind or two or more kinds of substances other than the lithium titanate powder. Other substances include, for example, carbon materials [pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), organic polymer compound combustors, carbon fibers], tin and tin compounds, silicon and silicon compounds. Is used.

[Electrode sheet]
The electrode sheet of the present invention is a sheet having a mixture layer containing an active material, a conductive agent and a binder on one side or both sides of a current collector, and is cut according to the design shape of a power storage device to form a positive electrode or a positive electrode sheet. Used as a negative electrode.
The electrode sheet of the present invention is an electrode sheet containing the lithium titanate powder of the present invention as an active material, and a mixture composed of a titanium raw material and a lithium raw material is fired, and the obtained fired product is granulated to have a dew point. The lithium titanate powder of the present invention obtained by heat-treating and cooling in a temperature range of 300 to 600 ° C. in an environment controlled to -20 ° C. or lower has a dew point of -20 ° C. or lower without substantially exposing it to the atmosphere. It is preferably produced by mixing with a conductive agent and a binder in a controlled environment. Here, substantially without exposure to the atmosphere means that the lithium titanate powder of the present invention is not exposed to the atmosphere at all, and the amount of water (25 ° C. to 350 ° C.) measured by the Karl Fischer method increases. It means exposure to the atmosphere to the extent that it does not occur.

The lithium titanate powder of the present invention contained in the electrode sheet as an active material is used in the process of being mixed with the conductive agent and the binder, and further in the process of forming the mixture layer. Subatomic particles are rarely completely maintained. In particular, since the lithium titanate powder of the present invention has an average 10% compressive strength of 0.1 MPa or more and 3 MPa or less and is relatively soft, the lithium titanate powder of the present invention undergoes an electrode sheet forming process. When it is contained in the electrode sheet, D50 is usually smaller than that before the electrode sheet is formed. Specifically, when the lithium titanate powder of the present invention is contained in the electrode sheet, the D50 of the lithium titanate powder of the present invention is preferably 1 μm or more and 30 μm or less, and more preferably 2 μm or more and 25 μm or less. Is.

[Power storage device]
The power storage device of the present invention is a device that stores and releases energy by utilizing intercalation and deintercalation of lithium ions to electrodes containing the active material, such as a hybrid capacitor and a lithium battery. Can be mentioned.

[Hybrid capacitor]
The hybrid capacitor includes an active material whose capacity is formed on the positive electrode by physical adsorption similar to that of the electrode material of an electric double layer capacitor such as activated carbon, and physical adsorption and intercalation and deintercalation such as graphite. It is a device that uses an active material whose capacity is formed by redox or an active material whose capacity is formed by redox such as a conductive polymer, and uses the active material material for a negative electrode. The active material is usually used in the form of an electrode sheet.

[Lithium battery]
The lithium battery of the present invention is a general term for a lithium primary battery and a lithium secondary battery. Further, in the present specification, the term lithium secondary battery is used as a concept including a so-called lithium ion secondary battery.

The lithium battery is composed of a positive electrode, a negative electrode, a non-aqueous electrolyte solution in which an electrolyte salt is dissolved in a non-aqueous solvent, and the like, and the active material material can be used as an electrode material. The active material is usually used in the form of an electrode sheet. This active material may be used as either a positive electrode active material or a negative electrode active material, but the case where it is used as a negative electrode active material will be described below.

<Negative electrode>
The negative electrode has a mixture layer containing a negative electrode active material (active material material of the present invention), a conductive agent, and a binder on one side or both sides of the negative electrode current collector. This mixture layer is usually in the form of an electrode sheet. In the case of a negative electrode current collector having pores such as a porous body, the pores have a mixture layer containing a negative electrode active material (active material material of the present invention), a conductive agent, and a binder.

The conductive agent for the negative electrode is not particularly limited as long as it is an electron conductive material that does not cause a chemical change. For example, natural graphite (scaly graphite, etc.), graphite such as artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, single-walled carbon nanotubes, and multi-walled carbon nanotubes. (Graphite layer is multi-layered concentric cylindrical) (non-fishbone), cup-stacked carbon nanotubes (fishbone), node-shaped carbon nanofibers (non-fishbone structure), platelet-type carbon nanofibers ( Examples thereof include carbon nanotubes (trump-shaped) and the like. Further, graphites, carbon blacks and carbon nanotubes may be appropriately mixed and used. There is no particular limitation, specific surface area of the carbon blacks is preferably 30~3000m ² / g, more preferably from 50~2000m ² / g. The specific surface area of graphites is preferably 30 to 600 m ² / g, and more preferably 50 to 500 m ² / g. The aspect ratio of the carbon nanotubes is 2 to 150, preferably 2 to 100, and more preferably 2 to 50.

Since the amount of the conductive agent added varies depending on the specific surface area of the active material and the type and combination of the conductive agents, optimization should be performed, but it is preferably 0.1 to 10% by mass in the mixture layer. More preferably, it is 0.5 to 5% by mass. If it is less than 0.1% by mass, the conductivity of the mixture layer cannot be ensured, and if it exceeds 10% by mass, the active material ratio decreases, and the unit mass of the mixture layer and the discharge capacity of the power storage device per unit volume are not good. Not suitable for high capacity because it is sufficient.

Examples of the binder for the negative electrode include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylpyrrolidone (PVP), a copolymer of styrene and butadiene (SBR), and a common weight of acrylonitrile and butadiene. Coalescence (NBR), carboxymethyl cellulose (CMC) and the like can be mentioned. Although not particularly limited, the molecular weight of polyvinylidene fluoride is preferably 20,000 to 200,000. From the viewpoint of ensuring the binding property of the mixture layer, it is preferably 25,000 or more, more preferably 30,000 or more, and further preferably 50,000 or more. From the viewpoint of ensuring conductivity without hindering contact between the active material and the conductive agent, it is preferably 150,000 or less. In particular, when the specific surface area of the active material is 10 m ² / g or more, the molecular weight is preferably 100,000 or more.

Since the amount of the binder added varies depending on the specific surface area of the active material and the type and combination of the conductive agent, it should be optimized, but it is preferably 0.2 to 15% by mass in the mixture layer. is there. From the viewpoint of enhancing the binding property and ensuring the strength of the mixture layer, it is preferably 0.5% by mass or more, more preferably 1% by mass or more, and further preferably 2% by mass or more. From the viewpoint of reducing the active material ratio and not reducing the unit mass of the mixture layer and the discharge capacity of the power storage device per unit volume, it is preferably 10% by mass or less, and more preferably 5% by mass or less.

Examples of the negative electrode current collector include aluminum, stainless steel, nickel, copper, titanium, calcined carbon, or a surface thereof coated with carbon, nickel, titanium, or silver. Further, the surface of these materials may be oxidized, or the surface of the negative electrode current collector may be made uneven by surface treatment. Examples of the form of the negative electrode current collector include a sheet, a net, a foil, a film, a punched body, a lath body, a porous body, a foam body, a fiber group, and a molded body of a non-woven fabric. Porous aluminum is preferable as the form of the negative electrode current collector. The porosity of the porous aluminum is 80% or more, 95% or less, and preferably 85% or more.

As a method for producing the negative electrode, a negative electrode active material (active material material of the present invention), a conductive agent and a binder are uniformly mixed in a solvent to form a paint, which is then applied onto the negative electrode current collector and dried. , Can be obtained by compressing. In the case of a negative electrode current collector having pores such as a porous body, a paint obtained by uniformly mixing a negative electrode active material (active material material of the present invention), a conductive agent and a binder in a solvent is used to empty the current collector. It can be obtained by press-fitting into the pores to fill it, or by immersing a current collector having pores in the paint, diffusing it into the pores, and then drying and compressing it.

As a method of uniformly mixing the negative electrode active material (active material material of the present invention), the conductive agent and the binder in the solvent to form a paint, for example, while the stirring rod rotates in a kneading container such as a planetary mixer. A revolving type kneader, a biaxial extrusion type kneader, a planetary stirring and defoaming device, a bead mill, a high-speed swirling mixer, a powder suction continuous dissolution / dispersion device, and the like can be used. Further, as the manufacturing process, the process may be divided according to the solid content concentration, and these devices may be used properly.

In order to uniformly mix the negative electrode active material (active material material of the present invention), the conductive agent and the binder in the solvent, the specific surface area of the active material, the type of the conductive agent, the type of the binder and the combination thereof are used. Since it is different, optimization should be performed, but a kneader of the type in which the stirring rod rotates while rotating in a kneading container such as a planetary mixer, a biaxial extrusion type kneader, a planetary stirring defoaming device, etc. are used. In this case, it is preferable to divide the process according to the solid content concentration as a manufacturing process, knead in a state where the solid content concentration is high, and then gradually reduce the solid content concentration to adjust the viscosity of the coating material. The solid content concentration is preferably 60 to 90% by mass, more preferably 70 to 90% by mass. If it is less than 60% by mass, a shearing force cannot be obtained, and if it exceeds 90% by mass, the load on the device becomes large, which is not suitable.

The mixing procedure is not particularly limited, but is a method of mixing the negative electrode active material, the conductive agent and the binder at the same time in the solvent, and a method of mixing the conductive agent and the binder in the solvent in advance and then the negative electrode active material. Examples include a method of additionally mixing the negative electrode active material slurry, a conductive agent slurry, and a binder solution prepared in advance, and the respective methods are mixed. Among these, in order to disperse uniformly, a method of mixing the conductive agent and the binder in a solvent in advance and then additionally mixing the negative electrode active material, or preparing a negative electrode active material slurry, a conductive agent slurry and a binder solution in advance. , A method of mixing each is preferable.

As the solvent, an organic solvent can be used. Examples of the organic solvent include alone or a mixture of two or more aprotic organic solvents such as N-methylpyrrolidone, dimethylacetamide, and dimethylformamide, and N-methylpyrrolidone is preferable.

When an organic solvent is used as the solvent, it is preferable to dissolve the binder in the organic solvent in advance before use.

<Positive electrode>
The positive electrode has a mixture layer containing a positive electrode active material, a conductive agent, and a binder on one side or both sides of the positive electrode current collector.

As the positive electrode active material, a material capable of occluding and releasing lithium is used. For example, as the active material, a composite metal oxide containing lithium containing cobalt, manganese, and nickel, a lithium-containing olivine-type phosphate, and the like are used. These positive electrode active materials can be used alone or in combination of two or more. Examples of such composite metal oxides include LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiCo _1-x Ni _x O ₂ (0.01 <x <1), LiCo _1/3 Ni _1/3 Mn. _{Examples thereof include 1/3} O ₂ , LiNi _1/2 Mn _3/2 O ₄ , and some of these lithium composite oxides may be replaced with other elements, and some of cobalt, manganese, and nickel may be Sn. , Mg, Fe, Ti, Al, Zr, Cr, V, Ga, Zn, Cu, Bi, Mo, La, etc., or a part of O is replaced with S or F. Alternatively, it can be coated with a compound containing these other elements. The lithium-containing olivine phosphate is selected from, for example, LiFePO ₄ , LiCoPO ₄ , LiNiPO ₄ , LiMnPO ₄ , LiFe _1-x M _x PO ₄ (M is Co, Ni, Mn, Cu, Zn, and Cd). There is at least one kind, and x is 0 ≦ x ≦ 0.5) and the like.

Examples of the conductive agent and the binder for the positive electrode include the same as those for the negative electrode. Examples of the positive electrode current collector include aluminum, stainless steel, nickel, titanium, calcined carbon, aluminum and stainless steel whose surface is surface-treated with carbon, nickel, titanium and silver. The surface of these materials may be oxidized, or the surface of the positive electrode current collector may be made uneven by surface treatment. Examples of the form of the current collector include a sheet, a net, a foil, a film, a punched body, a lath body, a porous body, a foam body, a fiber group, and a molded body of a non-woven fabric.

<Non-aqueous electrolyte>
The non-aqueous electrolyte solution is a solution in which an electrolyte salt is dissolved in a non-aqueous solvent. The non-aqueous electrolytic solution is not particularly limited, and various ones can be used.

As the electrolyte salt, one that dissolves in a non-aqueous electrolyte is used, and for example, _{an inorganic lithium salt such as LiPF 6} , LiBF ₄ , LiPO ₂ F ₂ , LiN (SO ₂ F) ₂ , LiClO ₄ or the like, LiN (SO _2). CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiCF ₃ SO ₃ , LiC (SO ₂ CF ₃ ) ₃ , LiPF ₄ (CF ₃ ) ₂ , LiPF ₃ (C ₂ F ₅ ) ₃ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₃ (iso-C ₃ F ₇ ) ₃ , LiPF ₅ (iso-C ₃ F ₇ ) and other lithium salts containing a chain-like alkyl fluoride group, and (CF ₂ ) ₂ ( SO ₂ ) ₂ NLi, (CF ₂ ) ₃ (SO ₂ ) ₂ NLi and other lithium salts containing cyclic fluorinated alkylene chains, bis [oxalate-O, O'] lithium borate and difluoro [oxalate-O, O'] Examples thereof include a lithium salt having an oxalate complex such as lithium borate as an anion. Among these, particularly preferable electrolyte salts are LiPF ₆ , LiBF ₄ , LiPO ₂ F ₂ , and LiN (SO ₂ F) ₂ , and the most preferable electrolyte salt is LiPF ₆ . These electrolyte salts can be used alone or in combination of two or more. In addition, a suitable combination of these electrolyte salts _{includes LiPF 6} , and at least one lithium salt selected from _{LiBF 4} , LiPO ₂ F ₂ , and LiN (SO ₂ F) ₂ is contained in the non-aqueous electrolyte solution. It is preferable that it is contained in.

The concentration in which all the electrolyte salts are dissolved and used is usually preferably 0.3 M or more, more preferably 0.5 M or more, still more preferably 0.7 M or more, based on the above-mentioned non-aqueous solvent. The upper limit thereof is preferably 2.5 M or less, more preferably 2.0 M or less, and even more preferably 1.5 M or less.

On the other hand, examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, chain esters, ethers, amides, phosphate esters, sulfones, lactones, nitriles, S = O bond-containing compounds and the like, and include cyclic carbonates. Is preferable. The term "chain ester" is used as a concept including a chain carbonate and a chain carboxylic acid ester.

As the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one (FEC), trans or Sis-4,5-difluoro-1,3-dioxolan-2-one (hereinafter collectively referred to as "DFEC"), vinylene carbonate (VC), vinylethylene carbonate (VEC), and 4-ethynyl-1. , 3-Dioxolan-2-one (EEC), one or more, including ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3. One or more selected from −dioxolan-2-one and 4-ethynyl-1,3-dioxolan-2-one (EEC) suppresses resistance increase, capacity decrease and gas generation during high temperature charge / discharge cycles of power storage devices. From the viewpoint, it is more preferable, and one or more of cyclic carbonates having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate and 2,3-butylene carbonate are further preferable. The proportion of the cyclic carbonate having an alkylene chain in the total cyclic carbonate is preferably 55% by volume to 100% by volume, more preferably 60% by volume to 90% by volume.

Therefore, as the non-aqueous electrolyte solution, ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one and 4-ethynyl-1, In a non-aqueous solvent containing one or more cyclic carbonates selected from 3-dioxolan-2-one, at least one lithium salt selected from _{LiPF 6} , LiBF ₄ , LiPO ₂ F ₂ , and LiN (SO ₂ F) _{2 is added.} It is preferable to use a non-aqueous electrolyte solution in which the containing electrolyte salt is dissolved, and the cyclic carbonate is a kind of cyclic carbonate having an alkylene chain selected from propylene carbonate, 1,2-butylene carbonate and 2,3-butylene carbonate. The above is more preferable.

_{In particular, LiBF 4} , LiPO ₂ F ₂ , and LiN having a total electrolyte salt concentration of 0.5 M or more and 2.0 M or less, _{containing at least LiPF 6} as the electrolyte salt, and 0.001 M or more and 1 M or less. It is preferable to use a non-aqueous electrolyte solution containing at least one lithium salt selected from (SO ₂ F) _2. When _{the ratio of lithium salts other than LiPF 6 in the} non-aqueous solvent is 0.001 M or more, the resistance increase, capacity reduction and gas generation suppression effect of the power storage device during the high temperature charge / discharge cycle are likely to be exhibited. When it is 0 M or less, there is little concern that the resistance of the power storage device increases during the high-temperature charge / discharge cycle, the capacity decreases, and the effect of suppressing gas generation decreases, which is preferable. The _{proportion of the lithium salt other than LiPF 6 in the} non-aqueous solvent is preferably 0.01 M or more, particularly preferably 0.03 M or more, and most preferably 0.04 M or more. The upper limit is preferably 0.8 M or less, more preferably 0.6 M or less, and particularly preferably 0.4 M or less.

Moreover, it is preferable that the non-aqueous solvent is mixed and used in order to achieve appropriate physical characteristics. The combinations are, for example, a combination of cyclic carbonate and chain carbonate, a combination of cyclic carbonate, chain carbonate and lactone, a combination of cyclic carbonate, chain carbonate and ether, and a combination of cyclic carbonate, chain carbonate and chain ester. Examples thereof include a combination of a cyclic carbonate, a chain carbonate and a nitrile, a combination of a cyclic carbonate, a chain carbonate and an S = O bond-containing compound, and the like.

Examples of the chain ester include one or more asymmetric chain carbonates selected from methyl ethyl carbonate (MEC), methyl propyl carbonate (MPC), methyl isopropyl carbonate (MIPC), methyl butyl carbonate, and ethyl propyl carbonate. One or more symmetrical chain carbonates selected from dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, and dibutyl carbonate, pivalic acid esters such as methyl pivalate, ethyl pivalate, and propyl pivalate. , One or more chain carboxylic acid esters selected from methyl propionate, ethyl propionate, propyl propionate, methyl acetate, and ethyl acetate (EA) are preferred.

Among the chain esters, a chain ester having a methyl group selected from dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, methyl propionate, methyl acetate and ethyl acetate (EA) is preferable. In particular, a chain carbonate having a methyl group is preferable.

When chain carbonate is used, it is preferable to use two or more kinds. Further, it is more preferable that both the symmetric chain carbonate and the asymmetric chain carbonate are contained, and it is further preferable that the content of the symmetric chain carbonate is higher than that of the asymmetric chain carbonate.

The content of the chain ester is not particularly limited, but it is preferably used in the range of 60 to 90% by volume with respect to the total volume of the non-aqueous solvent. If the content is 60% by volume or more, the viscosity of the non-aqueous electrolyte solution does not become too high, and if it is 90% by volume or less, the electrical conductivity of the non-aqueous electrolyte solution decreases, and during a high-temperature charge / discharge cycle of the power storage device. It is preferable to be in the above range because there is little possibility that the resistance increase, the capacity decrease, and the gas generation suppressing effect are reduced.

The volume ratio of the symmetrical chain carbonate to the chain carbonate is preferably 51% by volume or more, more preferably 55% by volume or more. The upper limit is more preferably 95% by volume or less, and further preferably 85% by volume or less. It is particularly preferable that the symmetric chain carbonate contains dimethyl carbonate. Further, the asymmetric chain carbonate is more preferably having a methyl group, and methyl ethyl carbonate is particularly preferable. In the above case, the resistance increase, capacity reduction, and gas generation suppression effect of the power storage device during the high temperature charge / discharge cycle are improved, which is preferable.

The ratio of cyclic carbonate to chain ester is from 10:90 to cyclic carbonate: chain ester (volume ratio) from the viewpoint of enhancing the resistance increase, capacity reduction and gas generation suppression effect of the power storage device during the high temperature charge / discharge cycle. 45:55 is preferable, 15:85-40:60 is more preferable, and 20:80 to 35:65 is particularly preferable.

<Lithium battery structure>
The structure of the lithium battery of the present invention is not particularly limited, and is a coin-type battery having a positive electrode, a negative electrode and a single-layer or multi-layer separator, and a cylindrical battery or a square having a positive electrode, a negative electrode and a roll-shaped separator. A type battery and the like can be mentioned as an example.

As the separator, an insulating thin film having a large ion transmittance and a predetermined mechanical strength is used. For example, polyethylene, polypropylene, cellulose paper, glass fiber paper, polyethylene terephthalate, polyimide microporous film and the like can be mentioned, and a multilayer film formed by combining two or more kinds can also be used. Further, the surface of these separators can be coated with resins such as PVDF, silicon resin and rubber resin, and particles of metal oxides such as aluminum oxide, silicon dioxide and magnesium oxide. The pore diameter of the separator may be in a range generally useful for batteries, and is, for example, 0.01 to 10 μm. The thickness of the separator may be in the range for a general battery, and is, for example, 5 to 300 μm.

Next, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples, and includes various combinations that can be easily inferred from the gist of the invention. In particular, the combination of solvents in the examples is not limited. The manufacturing conditions of Examples and Comparative Examples described below are summarized in Table 1.

[Example 1]
<Raw material preparation process>
Raw material obtained by weighing _{Li 2} CO ₃ (average particle size 4.6 μm) and anatase-type TiO ₂ (specific surface area 10 m ² / g) so that the atomic ratio of Li to Ti is 0.83. Ion-exchanged water was added to the powder so that the solid content concentration of the slurry was 41% by mass, and the mixture was stirred to prepare a raw material mixed slurry. Using a bead mill (manufactured by Willy et Bacoffen, type: Dynomill KD-20BC type, agitator material: polyurethane, vessel inner surface material: zirconia), beads made of zirconia (outer diameter: 0. 65 mm) was filled in the vessel in an amount of 80% by volume, and the process was performed at an agitator peripheral speed of 13 m / s and a slurry feed speed of 55 kg / hr while controlling the internal pressure of the vessel to be 0.02 to 0.03 MPa or less. Was wet-mixed and pulverized.

<Baking process>
The obtained mixed slurry is introduced into the core tube from the raw material supply side of the firing furnace using a rotary kiln type firing furnace equipped with an adhesion prevention mechanism (core tube length: 4 m, core tube diameter: 30 cm, external heating type). Then, it was dried in a nitrogen atmosphere and fired. At this time, the inclination angle of the core tube from the horizontal direction is 2 degrees, the rotation speed of the core tube is 20 rpm, the flow velocity of nitrogen introduced into the core tube from the fired product recovery side is 20 L / min, and the heating temperature of the core tube is set. The raw material supply side: 900 ° C., the central portion: 900 ° C., the fired product recovery side: 900 ° C., and the holding time of the fired product at 900 ° C. was 30 minutes.

<Post-treatment process>
Using a hammer mill (Dalton, AIIW-5 type), the fired product recovered from the fired product recovery side of the core tube has a screen opening of 0.5 mm, a rotation speed of 8,000 rpm, and a powder feed rate: It was crushed under the condition of 25 kg / hr. Ion-exchanged water was added to the crushed calcined powder so that the solid content concentration of the slurry was 30% by mass, and the mixture was stirred to prepare a calcined powder mixed slurry. This mixed slurry was spray-dried and granulated using a spray dryer (L-8i manufactured by Ogawara Kakoki Co., Ltd.) at an atomizer rotation speed of 25,000 rpm and an inlet temperature of 210 ° C. Next, the powder that has passed through the sieve is placed in an alumina saggar, and a mesh belt transport type continuous furnace equipped with a recovery box whose dew point is controlled to -20 ° C or lower at a temperature of 25 ° C is provided on the outlet side. Heat-treated for hours. The powder after heat treatment is cooled in the recovery box, separated into sieves (mesh roughness: 53 μm), the powder passed through the sieve is collected in an aluminum laminate bag, sealed, and then taken out from the recovery box and taken out from Example 1. The lithium titanate powder according to the above was obtained.

[Examples 2 to 5]
Lithium titanate powder according to Examples 2 to 5 was produced in the same manner as in Example 1 except that one or both of the heat treatment temperature and the heat treatment time in the post-treatment step were shown in Table 1.

[Example 6]
The raw material powder is dry-mixed for 30 minutes with a Henschel mixer type mixer (KAWATA, SUPERMIXER, SMV (G) -200), and the obtained mixed powder is filled in a high-purity alumina saggar to form a muffle type electric machine. The lithium titanate powder according to Example 6 was obtained in the same manner as in Example 1 except that it was fired in an air atmosphere at 850 ° C. for 1 hour using a furnace.

[Example 7]
Except for the fact that fine-grained anatase-type TiO ₂ (specific surface area 300 m ² / g) was used as the raw material, and that the maximum temperature in the firing process and the holding time at the maximum temperature were shown in Table 1 for firing. The lithium titanate powder according to Example 7 was produced in the same manner as in Example 1.

[Example 8]
Titanium according to Example 8 in the same manner as in Example 1 except that the maximum temperature in the firing step and the solid content concentration of the fired powder mixed slurry used for granulation in the post-treatment step are shown in Table 1. Lithium titanate powder was produced.

[Example 9]
The lithium titanate powder according to Example 8 was produced in the same manner as in Example 1 except that the solid content concentration of the calcined powder mixed slurry used for granulation in the post-treatment step was as shown in Table 1.

[Example 10]
The lithium titanate powder according to Example 10 was produced in the same manner as in Example 1 except that the dew points of the recovery box of the mesh belt transport type continuous furnace in the post-treatment step were as shown in Table 1.

[Comparative Example 1]
Lithium titanate powder according to Comparative Example 1 was produced in the same manner as in Example 1 except that the heat treatment temperature and the heat treatment time in the post-treatment step were shown in Table 1.

[Comparative Example 2]
Lithium titanate according to Comparative Example 2 as in Example 1 except that granulation was not performed after crushing the hammer mill in the post-treatment step and the heat treatment temperature and heat treatment time were shown in Table 1. The powder was produced.

[Comparative Example 3]
The raw material mixed slurry obtained in the same manner as in Example 1 is spray-dried and granulated using a spray dryer (L-8i manufactured by Okawara Kakoki Co., Ltd.) at an atomizer rotation speed of 25,000 rpm and an inlet temperature of 210 ° C. Then, using the granulated powder obtained by granulation, the granulated powder was prepared in the same manner as in Example 1 except that the maximum temperature and the holding time at the maximum temperature in the firing step were shown in Table 1. It was fired. The obtained calcined powder was post-treated under the same conditions as in Example 1 except that it was not crushed with a hammer mill and that granulation was not performed in the post-treatment step, and Comparative Example 3 The lithium titanate powder according to the above was produced.

[Comparative Example 4]
Lithium titanate powder according to Comparative Example 4 was produced in the same manner as in Comparative Example 3 except that the maximum temperature and the holding time at the maximum temperature in the firing step were shown in Table 1.

[Comparative Example 5]
Lithium titanate powder according to Comparative Example 5 was produced in the same manner as in Example 6 except that the maximum temperature and the holding time at the maximum temperature in the firing step were shown in Table 1.

[Comparative Example 6]
Lithium titanate powder according to Comparative Example 5 was produced in the same manner as in Example 7 except that the maximum temperature and the holding time at the maximum temperature in the firing step were shown in Table 1.

[Comparative Examples 7 and 8]
Lithium titanate powder according to Comparative Examples 7 and 8 was produced in the same manner as in Example 1 except that the solid content concentration of the calcined powder mixed slurry used for granulation in the post-treatment step was as shown in Table 1.

[Comparative Example 9]
Lithium titanate powder according to Comparative Example 9 was produced in the same manner as in Example 6 except that the solid content concentration of the calcined powder mixed slurry used for granulation in the post-treatment step was as shown in Table 1.

[Comparative Example 10]
Lithium titanate powder according to Comparative Example 10 was produced in the same manner as in Example 7 except that the solid content concentration of the calcined powder mixed slurry used for granulation in the post-treatment step was as shown in Table 1.

[Comparative Examples 11 and 12]
Lithium titanate powder according to Comparative Examples 11 and 12 was produced in the same manner as in Example 1 except that the dew point control of the recovery box of the mesh belt transport type continuous furnace in the post-treatment step was as shown in Table 1.

[Comparative Example 13]
The raw material was adjusted in the same manner as in Comparative Example 3, and the obtained granulated powder was calcined in the same manner as in Example 6 except that the holding times at the maximum temperature and the maximum temperature were shown in Table 1. The lithium titanate powder according to Comparative Example 13 was produced without performing the post-treatment step.

[Comparative Example 14]
Lithium titanate powder according to Comparative Example 14 was produced in the same manner as in Comparative Example 13 except that the maximum temperature in the firing step was shown in Table 1.

[Comparative Example 15]
Lithium titanate powder according to Comparative Example 15 was produced in the same manner as in Example 1 except that the heat treatment temperature in the post-treatment step was shown in Table 1.

[Comparative Example 16]
The granulated powder was calcined in the same manner as in Comparative Example 13 except that the maximum temperature in the calcining step was shown in Table 1. The obtained calcined powder was pulverized using ion-exchanged water and a bead mill, dried, and further pulverized with a hammer mill. After that, post-treatment was performed under the same conditions as in Comparative Example 3 except that the dew points of the recovery box of the mesh belt transport type continuous furnace were shown in Table 1 and the classification was not performed, and the present invention relates to Comparative Example 16. Lithium titanate powder was produced.

[Measurement of powder physical characteristics]
Various physical properties of the lithium titanate powders of Examples 1 to 10 and Comparative Examples 1 to 16 (hereinafter, may be referred to as lithium titanate powders of each example) were measured as follows. The measurement results are shown in Table 2.

なお、実施例３の水分量は、測定装置の検出限界以下であり、検出限界値を考慮すると、実質的に０ｐｐｍと判断できるものであった。

The water content in Example 3 was below the detection limit of the measuring device, and when the detection limit value was taken into consideration, it could be determined to be substantially 0 ppm.

<Measurement of BET specific surface area (m ² / g)>
^{For the BET specific surface area (m 2} / g) of the lithium titanate powder of each example and each comparative example, a fully automatic BET specific surface area measuring device (manufactured by Mountech Co., Ltd., trade name "Macsorb HM model-1208") was used. Then, 0.5 g of the measurement sample powder was weighed, placed in a Φ12 standard cell (HM1201-031), and measured by a one-point method using liquid nitrogen.

<Calculation of specific surface area equivalent diameter ( _{DBET)>
The specific surface area equivalent diameter (DBET} ) of the lithium titanate powder of each example and each comparative example was calculated from the following formula (1) on the assumption that all the particles constituting the powder were spheres having the same diameter. Here, _DBET is described by the specific surface area equivalent diameter (μm), ρ _S is the true density of lithium titanate (3.45 g / cc), and S is described by the above-mentioned <BET specific surface area (m ² / g)>. It is a BET specific surface area (m ² / g) obtained by measuring by the above-mentioned method.
D _BET = 6 / (ρ _S × S) (1)

<Calculation of D50 and _{Dmax>
The D50 and Dmax} of the lithium titanate powder of each example and each comparative example were calculated from the particle size distribution curve measured using a laser diffraction / scattering type particle size distribution measuring machine (Microtrac MT3300EXII manufactured by Nikkiso Co., Ltd.). .. Put 50 mg of the sample into a container containing 50 ml of ion-exchanged water as a measurement solvent, shake the container by hand until it can be visually recognized that the powder is uniformly dispersed in the measurement solvent, and store the container in the measurement cell. , Ultrasonic waves (30W, 3s) were applied with the ultrasonic device in the device. Further, the measurement solvent was added until the transmittance of the slurry was within an appropriate range (the range indicated by the green bar of the apparatus), and the particle size distribution was measured. From the obtained particle size distribution curve, D50 of the mixed powder, that is, the volume median particle size and D _max , that is, the volume maximum particle size were calculated. The calculated D50 / D _BET was calculated by dividing the calculated D 50 by the D _BET.

<Measurement of average compressive strength of secondary particles>
For the measurement of the average compressive strength (10% compressive strength and compressive fracture strength) of the secondary particles of lithium titanate powder in each example, a Shimadzu microcompression tester (MCT-) with a test force of "1 g or less measurement mode" added. 510) was used. Select "Compression test" as the test mode, and measure the 10% compression strength when the particles are compressed by 10% of the measured particle size and the compression fracture strength (crush strength) which is the strength when the secondary particles collapse. did. This device can arbitrarily select the surface detection point of the test data and the fracture point where the secondary particles are compressed and fractured (crushed) at the discretion of the measurer, but the surface detection point and the fracture point of the present application are automatically detected by the apparatus. Points were used. That is, the 10% compressive strength and the compressive fracture strength (crush strength) are the values automatically detected by the apparatus. The software of the test apparatus is Shimazu MCT test software Version 2.20, and the analysis software is Shimazu MCT analysis application Version 2.20. The measurement sample dispersed on the sample stage is observed with an optical microscope at an arbitrary location, and it can be determined that the particles do not overlap each other and clearly form secondary particles in the viewing range of the optical microscope. Particles were randomly selected, measured one by one, and the average value of 50 points was calculated, and the average value was defined as "average compressive strength of secondary particles (average 10% compressive strength and average compressive fracture strength)". .. The field of view of the optical microscope can be confirmed in real time on a personal computer by a CCD camera, and the particle size is measured on the length measurement screen and read as the measured particle size. The type of indenter was set to "FLAT50", and the length measurement mode was set to "single". Using the "soft sample measurement" mode, the test force was 4.90 mN, the load rate was 0.0100 mN / sec, and the load holding time was 5 seconds. The compression fracture strength "undetected" means a state in which a characteristic change in the fracture point is not detected even when the test force is applied up to 4.90 mN. In this measurement, all the samples whose compressive fracture strength was "undetected" were in a state where the compressive fracture strength was "undetected" in all the measurements at 50 points.

<Measurement of average circularity>
Circularity is an index of sphericity when a particle is projected on a two-dimensional plane, and is an index that replaces sphericity. In the present invention, for the secondary particles of the lithium titanate powder of each example, the value obtained by expressing the peripheral length of a perfect circle having the same area as the particle to be measured as a percentage with respect to the peripheral length of the particle to be measured is used. It was determined as the circularity of the particles, and the average value was taken as the average circularity. The perimeter of the particles to be measured was determined by image processing the SEM image, and the circularity was determined by the following formula (2). Fifty particles were randomly selected from a plurality of SEM images measured at arbitrary locations, and the average value of the circularity of the 50 randomly selected particles was taken as the average circularity.

Circularity = (perimeter of a perfect circle having the same area as the particle to be measured) / (perimeter of the particle to be measured) x 100 (%) (2)

<Measurement of water content by Karl Fischer method>
The water content of the lithium titanate powder in each example was controlled by a temperature of 25 ° C and a dew point of -20 ° C or less, and a Karl Fischer titer equipped with a moisture vaporizer (EV-2000 manufactured by Hiranuma Sangyo Co., Ltd.). (AQ-2200 manufactured by Hiranuma Sangyo Co., Ltd.), using dry nitrogen as a carrier gas, the measurement was carried out as follows. 1 g of lithium titanate powder of each example was put into the cell of the moisture vaporizer from the inlet, the lid of the cell was closed, and the measurement was started. As soon as the start button of the device was pressed, a heater having reached 200 ° C. rose to cover the cell and held it in this state for 1 hour. The amount of water generated from the start of measurement to the completion of holding at 200 ° C. is the amount of water measured by the Karl Fischer method (25 ° C. to 200 ° C.) (in this specification, the amount of water content of 25 ° C. to 200 ° C.). There is). Then, the cell temperature was raised from 200 ° C. to 350 ° C. in 15 minutes and maintained at 350 ° C. for 1 hour. Moisture generated from the start of temperature rise from 200 ° C to the completion of holding at 350 ° C is the amount of water (200 ° C to 350 ° C) measured by the Karl Fischer method (in this specification, the water content of 200 ° C to 350 ° C). It was measured as a quantity). The total amount of the water content (ppm) at 25 ° C. to 200 ° C. and the water content (ppm) at 200 ° C. to 350 ° C. was calculated as the water content (ppm) at 25 ° C. to 350 ° C.

<X-ray diffraction measurement>
In addition to the above measurements, the lithium titanate powder of each example was subjected to X-ray diffraction measurement by the following method. Specifically, as a measuring device, an X-ray diffractometer using CuKα rays (manufactured by Rigaku Co., Ltd., RINT-TTR-III type) was used. The measurement conditions for X-ray diffraction measurement are: measurement angle range (2θ): 10 ° to 90 °, step interval: 0.02 °, measurement time: 0.25 seconds / step, source: CuKα ray, tube. The voltage was 50 kV and the current was 300 mA.

_{Among the measured diffraction peaks, the main peak intensity of Li 4} Ti ₅ O ₁₂ (diffraction angle 2θ = 18.1 to 18.5 °) in the PDF card 00-049-0207 of ICDD (PDF2010) (111). ) Peak intensity corresponding to the diffraction peak attributable to the plane), main peak intensity of anatase-type titanium dioxide in PDF card 01-070-6826 (diffraction angle 2θ = 24.7 to 25.7 ° range (101)) (Peak intensity corresponding to the diffraction peak attributable to the surface), the main peak intensity of rutile type titanium dioxide in the PDF card 01-070-7347 ((110) surface within the range of diffraction angle 2θ = 27.2 to 27.6 °) (Peak intensity corresponding to the diffraction peak attributed to) and the peak intensity of Li ₂ TiO ₃ (corresponding to the diffraction peak attributed to the (-133) plane within the range of diffraction angle 2θ = 43.5 to 43.8 °). Peak intensity) was measured.

Then, _{when the main peak intensity of Li 4} Ti ₅ O ₁₂ was set to 100, the relative values of the peak intensities of the anatase-type titanium dioxide, rutile-type titanium dioxide, and Li ₂ TiO _{3 were calculated.} The lithium titanate powder of each example (Examples 1 to 10) had a relative value of 5 or less in all the peak intensities, and was a lithium titanate powder containing _{Li 4} Ti ₅ O _{12 as a main component.}

[Evaluation of battery characteristics]
Coin-type batteries and laminated batteries were produced using the lithium titanate powder of each example, and their battery characteristics were evaluated. The evaluation results are shown in Table 2.

<Manufacturing of negative electrode sheet>
The negative electrode sheet was prepared as follows in a room controlled at a room temperature of 25 ° C. and a dew point of −20 ° C. or lower. The lithium titanate powder of each example was taken out from the aluminum laminate bag in a room controlled at a temperature of 25 ° C. and a dew point of −20 ° C. or lower. The extracted lithium titanate powder of each example was mixed at a ratio of 90% by mass as an active material, acetylene black as a conductive agent at 5% by mass, and polyvinylidene fluoride as a binder at a ratio of 5% by mass as follows. The paint was made. After mixing polyvinylidene fluoride, acetylene black and 1-methyl-2-pyrrolidone solvent previously dissolved in 1-methyl-2-pyrrolidone solvent with a planetary stirring defoaming device, lithium titanate powder is added and the whole solid is added. The solvent was adjusted to have a component concentration of 64% by mass, and the mixture was mixed with a planetary stirring defoaming device. Then, a 1-methyl-2-pyrrolidone solvent was added to adjust the total solid content concentration to 50% by mass, and the mixture was mixed with a planetary stirring and defoaming device to prepare a coating material. The obtained paint was applied onto an aluminum foil and dried to prepare a negative electrode single-sided sheet used for a coin battery described later. Further, a paint was also applied to the opposite surface of the obtained negative electrode single-sided sheet and dried to prepare a negative electrode double-sided sheet to be used for a laminated battery described later.

<Preparation of positive electrode sheet>
Lamination described later by the same method as described in <Preparation of Negative Electrode Sheet> described above, including the ratio of active material, conductive agent and binder, except that lithium cobalt oxide powder was used as the active material. A positive electrode double-sided sheet used for a mold battery was produced.

<Preparation of electrolyte>
The electrolytic solution used for the battery for character evaluation was prepared as follows. Ethylene carbonate (EC): propylene carbonate (PC): methyl ethyl carbonate (MEC): dimethyl carbonate (DMC) = 10:20:20:50 in an argon box controlled at a temperature of 25 ° C. and a dew point of -70 ° C. or lower. A (volume ratio) non-aqueous solvent was prepared, and LiPF ₆ was dissolved as an electrolyte salt at a concentration of 1 M and LiPO ₂ F ₂ at a concentration of 0.05 M to prepare an electrolytic solution.

<Making coin batteries>
The negative electrode single-sided sheet prepared by the above method was punched into a circle having a diameter of 14 mm ^{, pressed at a pressure of 2} t / cm 2, and then vacuum dried at 120 ° C. for 5 hours to prepare an evaluation electrode. The prepared evaluation electrode and metallic lithium (molded into a circle having a thickness of 0.5 mm and a diameter of 16 mm) were opposed to each other via a glass filter (two layers of ADVANTEC GA-100 and Watman GF / C), and described above. A 2032 type coin-type battery was produced by adding and sealing the non-aqueous electrolyte solution prepared by the method described in <Preparation of Electrolyte Solution>.

For the electrode density of the negative electrode, measure the thickness and mass of the evaluation electrode, and subtract the thickness and mass of the current collector (circle with a diameter of 14 mm, 20 μm, 8.5 mg) from the mixture (negative electrode active material (book). The thickness and mass of the active material material of the invention), including the conductive agent and the binder) were obtained and calculated. The electrode D50 decomposes and removes acetylene black (conductive agent) and polyvinylidene fluoride (binding agent) by scraping the mixture from the negative electrode single-sided sheet and heat-treating the scraped mixture in air at 500 ° C. for 3 hours. The lithium titanate powder after the above was measured and obtained by the same method as described in _{<D50, D max> described above.} Table 2 shows the electrode density and the results of the electrode D50. The results of the ratio of the electrode D50 to the powder D50 (electrode D50 / powder D50 (μm / μm)) are also shown in the table.

<Manufacturing of laminated batteries>
The laminated battery was manufactured in a room controlled at a room temperature of 25 ° C. and a dew point of −20 ° C. or lower. The negative electrode double-sided sheet was ^{press-processed at a pressure of 2} t / cm 2, and then punched to prepare a negative electrode having a lead wire connecting portion. The positive electrode double-sided sheet was ^{press-processed at a pressure of 2} t / cm 2, and then punched to prepare a positive electrode having a lead wire connecting portion. The prepared negative electrode and positive electrode were vacuum dried at 150 ° C. for 12 hours. The positive and negative electrodes after vacuum drying are opposed to each other via a separator (manufactured by Ube Kosan: UP3085) and laminated, the lead wires of the aluminum foil are connected together with the positive and negative electrodes, the non-aqueous electrolytic solution is added, and the aluminum laminate is used. A laminated battery for evaluation was produced by vacuum sealing. At this time, the capacity of the battery was 800 mAh, and the ratio of the capacity of the negative electrode to the positive electrode (negative electrode capacity / positive electrode capacity ratio) was 1.1.

<Measurement of negative electrode unipolar capacity>
In a constant temperature bath at 25 ° C., a current of ^{0.2 mA / cm 2} is charged in the direction in which Li is stored in the evaluation electrode in the coin-type battery manufactured by the method described in <Manufacturing a coin battery> described above. was charged with a density up to 1V, after further charge current at 1V makes a constant-current constant-voltage charge for charging to a current density of 0.05 mA / cm ^2, to 2V at a current density of 0.2 mA / cm ² The constant current discharge to be discharged was performed for 3 cycles. The discharge capacity in the third cycle was used as the initial capacity, and the unipolar capacity per active material amount (mAh / g) and the unipolar capacity per mixture volume (mAh / cm ³ ) were determined.

<Measurement of resistance increase rate before and after cycle, capacity maintenance rate, and amount of gas generated in cycle test>
In a constant temperature bath at 25 ° C., the laminated battery manufactured by the method described in <Manufacturing a laminated battery> described above is subjected to constant current charging to charge up to 2.75 V with a current of 0.2 C, and then charged. A constant current discharge of 0.2 C to 1.4 V was repeated for 3 cycles. Then, after charging to a capacity of 50% of the discharge capacity in the third cycle (SOC50 (SOC: State of Charge)), the DC current resistance is measured and the initial resistance value of the laminated battery (hereinafter referred to as the initial resistance value). May be noted). Further, the volume of the laminated battery at this time was measured by the Archimedes method and used as the initial volume of the laminated battery (hereinafter, may be referred to as the initial volume).

Next, this laminated battery is subjected to constant current charging in a constant temperature bath at 45 ° C. with a current of 0.2 C to 2.75 V, and then discharged to 1.4 V with a current of 0.2 C. Aging was performed by performing constant current discharge once. The volume of the laminated battery at this time was measured by the Archimedes method and used as the volume after aging of the laminated battery (hereinafter, may be referred to as the volume after aging). By subtracting the initial volume from the volume after aging, the amount of gas generated by aging (hereinafter, may be referred to as aging generated gas) was determined. The results are shown in Table 3.

Next, the laminated battery was opened in a dry box having a temperature of 25 ° C. and a dew point of −70 ° C. or lower and vacuum-sealed to remove the gas generated by the aging from the laminated battery. Further, the volume of the laminated battery at this time was measured by the Archimedes method and used as the volume after aging and vacuum sealing of the laminated battery (hereinafter, may be referred to as the volume after aging and vacuum sealing).

Next, in a constant temperature bath at 60 ° C., a cycle test is performed in which a constant current charge of charging up to 2.75 V with a current of 1 C is performed, and then a constant current discharge of discharging to 1.4 V with a current of 1 C is repeated for 700 cycles. Was done. The capacity retention rate (%) was calculated by dividing the discharge capacity of the 700th cycle of the cycle test by the discharge capacity of the first cycle. The results are shown in Table 2.

After a 700-cycle cycle test, this laminated battery is subjected to constant current charging in a constant temperature bath at 25 ° C. with a current of 0.2 C to 2.75 V, and then 1. A constant current discharge of up to 4 V was repeated for 3 cycles. After that, the battery is charged to a capacity of 50% of the discharge capacity of the third cycle (SOC50), and then the DC current resistance is measured, and the resistance value after the cycle test of the laminated battery (hereinafter referred to as the resistance value after the cycle test) may be described. There is). Further, the volume of the laminated battery at this time was measured by the Archimedes method and used as the volume after the cycle test of the laminated battery (hereinafter, may be referred to as the volume after the cycle test). Then, the resistance value after the cycle test was divided by the initial resistance value to calculate the resistance increase rate before and after the cycle. Further, the volume after aging / vacuum sealing was subtracted from the volume after the cycle test to determine the amount of gas generated by the cycle test of 700 cycles (in this specification, it may be referred to as the amount of gas generated in the cycle test).

[Example 11]
Using the lithium titanate powder according to Example 1, porous aluminum was used for the current collector instead of the aluminum foil at the time of <manufacturing the negative electrode sheet> in [evaluation of battery characteristics]. A porous aluminum current collector (porosity 91%, pore diameter 300 μm) was immersed in the slurry prepared under the same conditions as described above, and the pressure was reduced (−0.1 MPa). After immersion, excess slurry adhering to the front and back surfaces of the porous aluminum current collector was removed with a silicon rubber spatula and dried to prepare a negative electrode of the porous aluminum current collector. Further, at the time of <manufacturing a coin battery>, the press working ^{was changed to a pressure of 2 t / cm 2 to} a pressure of 0.8 t / cm ² . In the calculation of the negative electrode density, instead of subtracting the thickness and mass of the current collector (circle with a diameter of 14 mm, 20 μm, 8.5 mg), only the mass of the current collector (circle with a diameter of 14 mm, 37 mg) was subtracted. Other than that, [evaluation of battery characteristics] was performed in the same manner as in Example 1. The results are shown in Table 2.

<Reference experiment example 1>
In the method for producing a laminated battery described in <Making a laminated battery>, the vacuum drying condition of the negative electrode was changed to 80 ° C. for 2 hours to produce a laminated battery. The manufactured laminated battery was aged by the same method as described in <Measurement of resistance increase rate before and after cycle, capacity retention rate, and amount of gas generated in cycle test>, and the amount of gas generated by aging (aging generated gas) was measured. I asked. The results are shown in Table 3 as Reference Experimental Examples 1-1 to 1-26 together with the above-mentioned Examples and Comparative Examples vacuum-dried at 150 ° C. for 12 hours.

Further, FIG. 1 shows the amount of gas generated during aging of a laminated battery produced by changing the vacuum drying condition of the negative electrode at 80 ° C. for 2 hours, and the amount of water at 25 ° C. to 200 ° C. measured with lithium titanate powder. , The relationship with the water content of 200 ° C. to 350 ° C. is shown. Further, FIG. 2 shows the amount of gas generated during aging of the laminated battery produced under the vacuum drying condition of the negative electrode at 150 ° C. for 12 hours, and the amount of water at 25 ° C. to 200 ° C. measured with lithium titanate powder. The relationship with the water content of 200 ° C. to 350 ° C. is shown.
From FIG. 1, it can be seen that the amount of gas generated by aging has a large correlation with the amount of water at 25 ° C to 200 ° C (that is, Comparative Examples 7, 12 to 14 having a high amount of water at 25 ° C to 200 ° C (reference experiment). It was found that the amount of gas generated by aging increased in Examples 1-17, 1-22 to 1-24)), and from FIG. 2, the electrodes were vacuum-dried at 150 ° C. for 12 hours to 25 ° C. to 200 ° C. It was found that the water content at ° C can be almost removed.

<Reference experiment example 2>
The lithium titanate powder of Example 1 was taken out from the aluminum laminate bag and stored in a constant temperature and humidity chamber under each condition (temperature, humidity, time) shown in Table 4, and then <the amount of water by the Karl Fischer method. The water content was measured by the same method as described in Measurement>. Further, using the lithium titanate powder stored under each condition, a laminated battery was produced by the same method as described in the method for producing a laminated battery described in <Production of laminated battery>. The battery characteristics of the produced laminated battery were evaluated by the same method as described in <Measurement of resistance increase rate before and after cycle, capacity retention rate, and amount of gas generated in cycle test>. The above results are shown in Table 4 together with Example 1 as Reference Experimental Examples 2-1 to 2-5.

From this result, when the lithium titanate powder is exposed to a normal indoor environment such as a temperature of 25 ° C. and a humidity of 40%, the water content gradually increases, and as a result, the high temperature charge / discharge cycle test is performed. It was found that not only the amount of generated gas increased, but also the capacity retention rate and the rate of increase in resistance before and after the cycle deteriorated.

Claims (14)

Lithium titanate powder containing Li ₄ Ti ₅ O ₁₂ as the main component.
Containing secondary particles in which primary particles made of lithium titanate are aggregated,
When the specific surface area equivalent diameter calculated from the specific surface area obtained by the BET method is D _BET and the volume median particle size is D50, the D _BET is 0.03 μm or more and 0.6 μm or less.
D50 is 3 μm or more and 40 μm or less,
The ratio of D50 to D _BET , D50 / D _BET (μm / μm), is 20 or more and 350 or less.
The amount of water (25 ° C to 350 ° C) measured by the Karl Fischer method is 600 ppm or less.
A lithium titanate powder for an electrode of a power storage device, characterized in that the average 10% compression strength of the secondary particles is 0.1 MPa or more and 3 MPa or less. The lithium titanate powder for an electrode of a power storage device according to claim 1, wherein the compressive fracture strength (crush strength) is not detected. The lithium titanate powder for an electrode of a power storage device according to claim 1 or 2, wherein the water content (200 ° C. to 350 ° C.) measured by the Karl Fischer method is 150 ppm or less. The lithium titanate powder for an electrode of a power storage device according to any one of claims 1 to 3, wherein the D _max is 53 μm or less when the maximum volume particle size is D _max. The lithium titanate powder for an electrode of a power storage device according to any one of claims 1 to 4, wherein the average circularity of the secondary particles is 90% or more. The lithium titanate powder for an electrode of a power storage device according to any one of claims 1 to 5, wherein the average 10% compression strength of the secondary particles is 0.1 MPa or more and 1 MPa or less. An active material material containing lithium titanate powder for electrodes of a power storage device according to any one of claims 1 to 6. An electrode sheet of a power storage device produced by using the active material according to claim 7. A power storage device including the electrode sheet according to claim 8. A lithium ion secondary battery manufactured by using the active material according to claim 7. A hybrid capacitor produced by using the active material according to claim 7. Selected from ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 4-fluoro-1,3-dioxolan-2-one and 4-ethynyl-1,3-dioxolan-2-one. Non-aqueous electrolysis in which an electrolyte salt containing at least one lithium salt selected from _{LiPF 6} , LiBF ₄ , LiPO ₂ F ₂ , and LiN (SO ₂ F) ₂ is dissolved in a non-aqueous solvent containing one or more cyclic carbonates. The power storage device according to claim 9, wherein a liquid is used. The non-aqueous electrolyte solution has a total electrolyte salt concentration of 0.5 M or more and 2.0 M or less, _{contains at least LiPF 6 as the electrolyte salt, and further contains LiBF 4} and LiPO _{2 of} 0.001 M or more and 1.0 M or less. The power storage device according to claim 12, wherein the non-aqueous electrolyte solution contains at least one lithium salt selected from F ₂ and LiN (SO ₂ F) _2. The non-aqueous electrolyte solution contains one or more symmetrical chain carbonates selected from dimethyl carbonate, diethyl carbonate, dipropyl carbonate, and dibutyl carbonate, and methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, and methyl butyl. The power storage device according to claim 12 or 13, further comprising one or more asymmetric chain carbonates selected from carbonates and ethylpropyl carbonates.

Images